U.S. patent number 7,990,070 [Application Number 12/479,312] was granted by the patent office on 2011-08-02 for led power source and dc-dc converter.
Invention is credited to Louis Robert Nerone.
United States Patent |
7,990,070 |
Nerone |
August 2, 2011 |
LED power source and DC-DC converter
Abstract
Isolated LED power sources and DC-DC converters therefor are
presented in which the DC-DC converter includes a self-oscillating
inverter driving an output rectifier for operating an LED array of
one or more LEDs, where the inverter uses a control transformer
with core having a Curie temperature set to a maximum operating
temperature of one or more power supply components to reduce
inductances of secondary windings in the inverter oscillation
circuitry to lower the power supplied to the load so as to prevent
the inverter from overheating.
Inventors: |
Nerone; Louis Robert
(Brecksville, OH) |
Family
ID: |
42288621 |
Appl.
No.: |
12/479,312 |
Filed: |
June 5, 2009 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20100308751 A1 |
Dec 9, 2010 |
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Current U.S.
Class: |
315/247; 315/224;
315/291; 315/308 |
Current CPC
Class: |
H02M
1/32 (20130101); H02M 1/4225 (20130101); Y02B
70/10 (20130101); Y02B 70/126 (20130101) |
Current International
Class: |
H05B
41/16 (20060101); H05B 41/24 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0 340 049 |
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Nov 1989 |
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EP |
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0 479 352 |
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Apr 1992 |
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EP |
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1868284 |
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Dec 2007 |
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EP |
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61221581 |
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Oct 1986 |
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JP |
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2216796 |
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Aug 1990 |
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JP |
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9506350 |
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Mar 1995 |
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WO |
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2008055545 |
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May 2008 |
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WO |
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Other References
PCT/US2010/31968, Search Report and Written Opinion, Jul. 21, 2010.
cited by other.
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Primary Examiner: Tran; Anh
Attorney, Agent or Firm: Fay Sharpe LLP
Claims
The following is claimed:
1. An LED power source for operating an LED array of one or more
LEDs, the power source comprising: an input rectifier operative to
receive an input AC voltage signal and to produce an initial DC
voltage; and a DC-DC converter which is operatively coupled to the
input rectifier to receive the initial DC voltage, the DC-DC
converter comprising: a supply-side ground connection, a load-side
ground connection electrically isolated from the supply side
ground, a self-oscillating inverter operatively coupled to the
supply-side ground connection and receiving the initial DC voltage,
the inverter operative to produce an intermediate AC signal, and an
output rectifier operatively coupled to receive the intermediate AC
signal from the inverter and being coupled with the load-side
ground connection, the output rectifier operative to produce an
output DC voltage to supply power to the LED array; wherein the
inverter further comprises: a feedback transformer comprising: a
primary winding, and a first and second secondary winding
operatively connected to the primary winding of the feedback
transformer; a resonant circuit comprising: a signal-to-center
capacitance operatively connected to the positive rail of the
initial DC voltage, a center-to-ground capacitance operatively
connected between the signal-to-center capacitance and the
supply-side ground connection, and an inductance comprising the
primary winding of the feedback transformer operatively connected
to the junction of the signal-to-center capacitance and the
center-to-ground capacitance; a first gate control circuit
comprising: an inductance comprising the first secondary winding of
the feedback transformer operatively connected to the inductance of
the resonant circuit, a capacitance operatively connected to the
inductance of the first gate control circuit, and a resistance
operatively connected to the capacitance of the first gate drive
circuit; a second gate control circuit comprising: an inductance
comprising the second secondary winding of the feedback transformer
operatively connected to the supply-side ground connection, a
capacitance and resistance in parallel operatively connected to the
inductance of the second gate control circuit, and a resistance
operatively connected to the parallel capacitance-resistance of the
second gate drive circuit; and first and second switches
operatively connected in series between the positive rail of the DC
voltage and the supply-side ground connection, and the junction of
the first and second switches is connected to the inductance of the
resonant circuit, the first and second switches being controlled by
the first and second gate control circuits respectively; wherein
the primary winding of the feedback transformer induces a voltage
in the inductance of the first and second gate control circuits
proportional to the instantaneous rate of change of current in the
resonant circuit, the first and second gate control circuits
alternatively switching the first and second switches operating to
excite the resonant circuit.
2. The LED power source of claim 1, further comprising a boost
converter operatively coupled between the input rectifier and the
DC-DC converter, the boost converter receiving the initial DC
voltage from the input rectifier and amplifying the initial DC
voltage to provide an intermediate DC voltage, the inverter
converting the intermediate DC voltage to produce the intermediate
AC signal.
3. The LED power source of claim 2, wherein the boost converter
further comprises a power factor correction controller to control a
power factor of the power source.
4. An LED power source for operating an LED array of one or more
LEDs, the power source comprising: an input rectifier operative to
receive an input AC voltage signal and to produce an initial DC
voltage; and a DC-DC converter which is operatively coupled to the
input rectifier to receive the initial DC voltage, the DC-DC
converter comprising: a supply-side ground connection, a load-side
ground connection electrically isolated from the supply side
ground, a self-oscillating inverter operatively coupled to the
supply-side ground connection and receiving the initial DC voltage,
the inverter operative to produce an intermediate AC signal, and an
output rectifier operatively coupled to receive the intermediate AC
signal from the inverter and being coupled with the load-side
ground connection, the output rectifier operative to produce an
output DC voltage to supply power to the LED array; wherein the
DC-DC converter further comprises an inverter controller
operatively coupled to the inverter to modify a switching frequency
of the inverter to control a frequency of the intermediate AC
signal to control power supplied to the LED array based at least in
part on a sensed power drawn by the LED array.
5. The LED power source of claim 4, wherein the inverter controller
is electrically isolated from the inverter.
6. The LED power source of claim 4, wherein the DC-DC converter
further comprises a control transformer comprising: a primary
winding operatively coupled to and controlled by the inverter
controller; a secondary winding operative to control the switching
frequency of the inverter; and a core operatively coupled to the
primary winding and the secondary windings, the core comprising a
core material having a Curie temperature associated with a maximum
operating junction temperature of a component in the power source,
operative to become paramagnetic when the temperature of the core
exceeds the Curie temperature of the core; the control transformer
being operative to reduce the inductance of the secondary windings
to lower the power supplied to the LED array to prevent the
component of the power source from overheating.
7. The LED power source of claim 4, wherein the inverter further
comprises: a resonant circuit which generates an oscillation
signal, including a resonant inductance and a resonant capacitance;
first and second switches operatively connected together at a
common node to receive the oscillation signal determining the
switching rate of the first and second switches; a first gate
control circuit controlling the first switch, the first gate
control circuit comprising an inductance operatively connected
between the junction of the first and second switches and the gate
of the first switch and being coupled to the resonant inductance;
and a second gate control circuit controlling the second switch,
the second gate control circuit comprising an inductance
operatively connected between the supply-side ground connection and
the gate of the second switch and being coupled to the resonant
inductance; wherein the first and second switches induce an AC
current in the resonant circuit.
8. An isolated, self-oscillating DC-DC converter comprising: a
supply-side ground connection; a load-side ground connection
electrically isolated from the supply side ground connection; a
self-oscillating inverter operatively coupled to the supply-side
ground connection and receiving an initial DC voltage, the inverter
operative to produce an intermediate AC signal; an output rectifier
operatively coupled to receive the intermediate AC signal from the
inverter and being coupled with the load-side ground connection,
the output rectifier operative to produce an output DC voltage to
supply power to a load; and an inverter controller operatively
coupled to the inverter to modify a switching frequency of the
inverter to control a frequency of the intermediate AC signal to
control power supplied to a load based at least in part on a sensed
power drawn by the load.
9. The DC-DC converter of claim 8, wherein the inverter controller
is electrically isolated from the inverter.
10. The DC-DC converter of claim 8, further comprising a control
transformer comprising: a primary winding operatively coupled to
and controlled by the inverter controller; a secondary winding
operative to control the switching frequency of the inverter; and a
core operatively coupled to the primary winding and the secondary
windings, the core comprising a core material having a Curie
temperature associated with a maximum operating junction
temperature of at least one component in the power source the core
being operative to become paramagnetic when a temperature of the
core exceeds the Curie temperature of the core; the control
transformer being operative to reduce the inductance of the
secondary windings to lower the power supplied to the LED array to
prevent the component of the power source from overheating.
11. The DC-DC converter of claim 8, further comprising: a resonant
circuit which generates an oscillation signal, including a resonant
inductance and a resonant capacitance; first and second switches
operatively connected together at a common node to receive the
oscillation signal determining the switching rate of the first and
second switches; a first gate control circuit controlling the first
switch, the first gate control circuit comprising: an inductance
operatively connected between the junction of the first and second
switches and the gate of the first switch and being coupled to the
resonant inductance; and a second gate control circuit controlling
the second switch, the second gate control circuit comprising: an
inductance operatively connected between the supply-side ground
connection and the gate of the second switch and being coupled to
the resonant inductance; wherein the first and second switches
induce an AC current in the resonant circuit.
12. A self-oscillating DC-DC converter comprising: a
self-oscillating inverter receiving the initial DC voltage, the
inverter operative to produce an intermediate AC signal; an output
rectifier operatively coupled to receive the intermediate AC signal
from the inverter, the output rectifier operative to produce an
output DC voltage to supply power to a load; and a control
transformer comprising: a primary winding operatively coupled to
and controlled by an inverter controller; a secondary winding
operative to control the switching frequency of the inverter; and a
core operatively coupled to the primary winding and the secondary
windings, the core comprising a core material having a Curie
temperature associated with a maximum operating junction
temperature of at least one component in the inverter and operative
to become paramagnetic when a temperature of the core exceeds the
Curie temperature of the core; the control transformer being
operative to reduce the inductance of the secondary windings to
lower the power supplied to the LED array to prevent the component
of the power source from overheating.
13. The self-oscillating DC-DC converter of claim 12, further
comprising an inverter controller operatively coupled to the
inverter, to modify a switching frequency of the inverter, to
control a frequency of the intermediate AC signal to control power
supplied to the LED array based at least in part on a sensed power
drawn by the LED array.
14. The self-oscillating DC-DC converter of claim 12, wherein the
inverter further comprises: a resonant circuit which generates an
oscillation signal, including a resonant inductance and a resonant
capacitance; first and second switches operatively connected
together at a common node to receive the oscillation signal
determining the switching rate of the first and second switches; a
first gate control circuit controlling the first switch, the first
gate control circuit comprising an inductance operatively connected
between the junction of the first and second switches and the gate
of the first switch and being coupled to the resonant inductance;
and a second gate control circuit controlling the second switch,
the second gate control circuit comprising an inductance
operatively connected between the supply-side ground connection and
the gate of the second switch and being coupled to the resonant
inductance; wherein the first and second switches induce an AC
current in the resonant circuit.
15. The LED power source of claim 1, wherein the DC-DC converter
further comprises an inverter controller operatively coupled to the
inverter, to modify a switching frequency of the inverter, to
control a frequency of the intermediate AC signal to control power
supplied to the LED array based at least in part on a sensed power
drawn by the LED array.
16. The LED power source of claim 15, wherein the inverter
controller is electrically isolated from the inverter.
17. The DC-DC converter of claim 8, further comprising a boost
converter operatively coupled between an input rectifier and the
DC-DC converter, the boost converter receiving the initial DC
voltage from the input rectifier and amplifying the initial DC
voltage to provide an intermediate DC voltage, the inverter
converting the intermediate DC voltage to produce the intermediate
AC signal.
18. The DC-DC converter of claim 17, wherein the boost converter
further comprises a power factor correction controller to control a
power factor of the DC-DC converter.
19. The DC-DC converter of claim 9, further comprising a boost
converter operatively coupled between an input rectifier and the
DC-DC converter, the boost converter receiving the initial DC
voltage from the input rectifier and amplifying the initial DC
voltage to provide an intermediate DC voltage, the inverter
converting the intermediate DC voltage to produce the intermediate
AC signal.
20. The DC-DC converter of claim 18, wherein the boost converter
further comprises a power factor correction controller to control a
power factor of the DC-DC converter.
Description
BACKGROUND OF THE DISCLOSURE
Light emitting diodes (LEDs) are becoming more and more popular for
lighting and signaling applications, in which multiple LEDs are
formed into an array and powered to emit light. LED arrays are
typically supplied with DC current with the amount of supplied
power controlling the array brightness. In many applications, it is
desirable to maintain electrical isolation between the LED array
and the input power supply, such as where installers may ground the
LED array to earth ground. Moreover, it is desirable to avoid
thermal excesses in the LED power source. A need therefore exists
for improved LED power sources and DC-DC converters that provide
isolation and self-protection against overheating.
SUMMARY OF THE DISCLOSURE
An LED power source is provided along with a self oscillating DC-DC
converter therefor, which can be employed for operating an LED
array. The power source includes an input rectifier which receives
an input AC voltage signal and produces an initial DC voltage
received by the DC-DC converter. In one embodiment, the power
source includes a boost converter operatively coupled between the
input rectifier and the DC-DC converter, which may have a power
factor correction controller to control a power factor of the power
source. The DC-DC converter has isolated supply-side and load-side
ground connections as well as a self-oscillating inverter and an
output rectifier. The inverter is coupled with the supply-side
ground connection and receives the initial DC voltage directly or
indirectly from the input rectifier and produces an intermediate AC
signal. The output rectifier is coupled with the load-side ground
connection and receives the intermediate AC signal from which it
produces an output DC voltage to supply power to the LED array. The
DC-DC converter may further include an inverter controller that
modifies the inverter frequency to control the supplied power based
at least in part on sensed output power draw, where the inverter
controller is electrically isolated from the inverter in some
embodiments. The DC-DC converter may be controlled using a control
transformer with a primary winding operatively coupled to and
controlled by the inverter controller, as well as secondary
windings that control the inverter switching frequency. The
transformer in some embodiments has a core made of a material
having a Curie temperature associated with a maximum operating
temperature of a component in the power source, where the control
transformer reduces the inductance of the secondary windings when
the core temperature exceeds the Curie temperature to prevent the
power source from overheating.
BRIEF DESCRIPTION OF THE DRAWINGS
One or more exemplary embodiments are set forth in the following
detailed description and the drawings, in which:
FIG. 1 is a schematic diagram illustrating an exemplary LED power
source having a self-oscillating isolated DC-DC converter for
powering an LED array;
FIG. 2 is a schematic diagram illustrating details of an exemplary
self-oscillating isolated DC-DC converter; and
FIG. 3 is a side elevation view illustrating an exemplary control
transformer with a core having a Curie temperature Tc set for
thermal protection of the system and DC-DC converter of FIGS. 1 and
2.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to the drawings, where like reference numerals are
used to refer to like elements throughout, and wherein the various
features are not necessarily drawn to scale. FIG. 1 illustrates an
LED power source 102 including a rectifier 110 receiving input
power from an AC input 104, where the rectifier can be active or
passive or the power source 102 can alternatively be supplied with
DC input power with the rectifier omitted. The rectifier 110 has an
output 112 providing a rectified DC voltage to a switching type
DC-DC converter 120, which includes various switching devices
operated by suitable control signals (not shown). In one
embodiment, the converter 120 is a boost converter with a power
factor control (PFC) component 121 to control the power factor of
the ballast 102. In certain embodiments, the initial converter 120
may be omitted, with the rectifier (or external DC supply)
providing the DC voltage 122.
The LED power source 102 further includes a self-oscillating
isolated DC-DC converter 200 that is comprised of an inverter 210
receiving the DC voltage 122 and generating an intermediate AC, and
a controller 220 that controls the inverter 210. The inverter 210
is transformer coupled to provide isolated AC power to an output
rectifier 230. The output rectifier converts the intermediate AC to
provide a DC power output 202 for driving an LED array load 130,
which may include any number of LEDs 132 arranged in any suitable
series and/or parallel configuration. The exemplary controller 220
senses one or more conditions at the output rectifier 230 and
selectively modifies operation of the inverter 210 accordingly.
Referring also to FIG. 2, further details of an embodiment of a
self-oscillating isolated DC-DC converter 210 are illustrated. The
exemplary DC-DC converter 200 of FIG. 2 may be advantageously
employed in the LED power source 102 of FIG. 1 above, and may be
employed in any system in which isolated DC-DC conversion is
required. As shown in FIG. 2, the DC-DC converter 200 includes an
inverter 210 having terminals 210a and 210b that receive DC power
from the boost converter 120 or other preceding DC supply. The
inverter 210, moreover, includes a resonant circuit 213 and a pair
of resonance controlled switching devices Q1 and Q2, in one
example, n-type MOSFETs although any suitable switching devices may
be employed. The inverter receives DC input voltage via the
terminals 210a and 210b and this input DC is selectively switching
by the switching devices Q1 and Q2 coupled in series between a
positive voltage node DC+ and a negative node coupled to a first
circuit ground GND1. In operation, the selective switching of the
switches Q1 and Q2 operates to generate a square wave at an
inverter output node 211 which in turn excites the resonant circuit
213 to thereby drive a high frequency bus 212.
The converter 200 includes transformers for power and control
isolation, as well as for self-oscillation. These include a first
transformer T1 with a winding T1A in the resonant circuit 213 and a
winding T1B in the output rectifier circuit 230, a second
transformer T2 having a first winding T2A in series with the
winding T1A in the resonant circuit 213 and second and third
windings T2B and T2C in switch control circuits associated with the
first and second switching devices Q1 and Q2, respectively, as well
as a third transformer T3 with a first winding T3A in the
controller 220 and second and third windings T3B and T3C in the
switch control circuits associated with Q1 and Q2. In operation,
the first transformer T1 provides AC power from the high frequency
bus 212 to the secondary winding T1B of the output rectifier 230,
and the secondary current from T1B is rectified to provide output
power at terminals 202a and 202b for driving the LED array 130 with
power isolated from the first circuit ground GND1, where the
negative output terminal 202b provides an output ground GND2 that
is isolated from the first circuit ground GND1 via the first
transformer T1. The first winding T2A of the second transformer
operates as a primary in the resonant circuit 213 and the secondary
windings T2B and T2C are connected in the gate drive circuits for
Q1 and Q2, respectively for oscillatory actuation of the switches
according to the resonance of the circuit 213. The third
transformer T3 is used by the controller 220 to selectively control
the inductance of the gate drive circuits for closed loop operation
of the converter 200, in one example, to control the LED array
output current.
The high frequency bus is generated at the node 212 by the inverter
210 and the resonant circuit 213, which includes a resonant
inductance (e.g., the series connected windings T1A and T2A) as
well as an equivalent resonant capacitance including the equivalent
of capacitors C1 and C2 connected in series between the DC+ and
GND1 nodes, with a center node at the high frequency bus 212. The
inverter 210 also includes a clamping circuit formed by clamping
diodes D1 and D2 individually coupled in parallel with the
capacitances C1 and C2, respectively. The switches Q1 and Q2 are
alternately activated to provide a square wave of amplitude VDC/2
at a common inverter output node 211 (e.g., half the DC bus voltage
across the terminals 122a and 122b). The square wave output of the
inverter 210 excites the resonant circuit 213. Gate or control
lines 214 and 216 include resistances R1 and R2 for providing
control signals to the gates of the inverter switches Q1 and Q2,
respectively.
The switch gating signals are generated using first and second gate
drive circuits 221 and 222, respectively, with the first drive
circuit 221 coupled between the inverter output node 211 and a
first circuit node 218, and the second drive circuit 222 coupled
between the circuit ground GND1 and node 216. The drive circuits
221 and 222 respectively include the first and second driving
inductors T2B and T2C which are secondary windings mutually coupled
to the resonant inductor T2A of the resonant circuit 213 to induce
voltage in the driving inductors T2B and T2C proportional to the
instantaneous rate of change of current in the resonant circuit 213
for self-oscillatory operation of the inverter 210. In addition,
the drive circuits 221 and 222 include the secondary inductors T3B
and T3C serially connected to the respective first and second
driving inductors T2B and T2C and the gate control lines 214 and
216, where the controller 220 can change the oscillatory frequency
of the inverter 210 by varying the inductance of the windings T3B
and T3C through control of the current through the primary winding
T3A.
In operation, the gate drive circuits 221 and 222 maintain Q1 in an
"ON" state for a first half of a cycle and the switch Q2 "ON" for a
second half of the cycle to generate a generally square wave at the
output node 211 for excitation of the resonant circuit 213. The
gate to source voltages Vgs of the switching devices Q1 and Q2 in
one embodiment are limited by bi-directional voltage clamps Z1, Z2
and Z3, Z4 (e.g., back-to-back Zener diodes) coupled between the
respective switch sources and the gate control lines 214 and 216.
In this embodiment, the individual bi-directional voltage clamp Z1,
Z2 and Z3, Z4 cooperate with the respective inductor T3B and T3C to
control the phase angle between the fundamental frequency component
of voltage across the resonant circuit 213 and the AC current in
the resonant inductor T2A.
To start the converter 200, series coupled resistors R3 and R4
across the input terminals 122a and 122b cooperate with a resistor
R6 (coupled between the inverter output node 211 and the circuit
GND1) to initiate regenerative operation of the gate drive circuits
221 and 222. In addition, the inverter switch control circuitry
includes capacitors C3 and C4 coupled in series with the windings
T3B and T3C, respectively. Upon application of DC power, C3 is
charged from the positive DC input via R3, R4 and R6. During this
time, a resistor R5 shunts the capacitor C4 to prevent C4 from
charging and thereby prevents concurrent activation of Q1 and Q2.
Since the voltage across C3 is initially zero, the series connected
inductors T2B and T3B act as a short circuit due to a relatively
long time constant for charging of the capacitor C3. Once C3
charges up to the threshold voltage of the Vgs of Q1, (e.g., 2-3
volts in one embodiment), Q1 turns ON and a small bias current
flows through Q1. This current biases Q1 in a common drain, Class A
amplifier configuration having sufficient gain to allow the
combination of the resonant circuit 213 and the gate control
circuit 221 to produce a regenerative action to begin oscillation
of the inverter 210 at or near the resonant frequency of the
network including C3 T3B and T2B, which is above the natural
resonant frequency of the resonant circuit 213. As a result, the
resonant voltage seen at the high frequency bus node 212 lags the
fundamental of the inverter output voltage at node 211, thereby
facilitating soft-switching operation of the inverter 210. The
inverter 210 therefore begins operation in a linear mode at startup
and transitions into switching Class D mode.
In steady state operation of the LED power source 102 circuit, the
square wave voltage at the inverter output node 211 has an
amplitude of approximately one-half of the voltage of the positive
terminal 122a (e.g. Vdc/2), and the initial bias voltage across C3
drops. In the illustrated inverter a first network 224 including
the capacitor C3 and inductor T3B and a second network 226
including the capacitor C4 and inductor T3C are equivalently
inductive with an operating frequency above the resonant frequency
of the first and second networks 224, 226. In steady state
oscillatory operation, this results in a phase shift of the gate
circuit to allow the current flowing through the inductor T2A to
lag the fundamental frequency of the voltage produced at the
inverter output node 211, thus facilitating soft-switching of the
inverter 210 during the steady-state operation. The output voltage
of the inverter 210 in one embodiment is clamped by serially
connected clamping diodes D1, D2 to limit high voltage seen by the
capacitors C1 and C2. As the inverter output voltage at node 211
increases, the clamping diodes D1, D2 start to clamp, preventing
the voltage across the capacitors C1 and C2 from changing sign and
limiting the output voltage to a value that prevents thermal damage
to components of the inverter 210.
In steady state operation, therefore, the inverter 210 provides a
high frequency bus at the common node 211 while maintaining the
soft switching condition for Q1 and Q2. The high frequency current
flowing through the primary winding T1A of the resonant circuit 213
is transformer coupled to the secondary winding T1B that drives a
passive full wave rectifier bridge which includes diodes D7, D8,
D9, and D10 in the output rectifier 230. Other forms of output
rectifiers can be used, including active or passive, full or
half-wave rectifiers, with or without filtering components. The
exemplary output rectifier 230 includes an output filter
capacitance C8 operative to smooth the rectified DC voltage from
the diodes bridge D7-D10, and the resulting DC output voltage is
provided at rectifier output terminals 202a and 202b, with the
negative output terminal 202b forming an output ground GND2 as
shown in FIG. 2. The output rectifier 230 also includes a sense
resistance R11 coupled between the lower leg of the rectifier
D7-D10 and the negative output terminal 202b, where the voltage
across R11 is proportional to the DC current provided to the LED
array 130 (or to other load) connected to the output terminals
202.
The controller circuit 220 senses this load current signal and
operates to vary/control the inductance of the inverter windings
T3B and T3C, and hence the operating frequency of the inverter 210
(by changing the loading seen by the tertiary winding T3A. In
particular, as the frequency of the inverter 210 is decreased, the
output current provided to the LED (sensed via resistor R11) will
increase, and vice versa. The inverter frequency, moreover,
decreases with decreased loading of T3A. Thus, the exemplary
controller 220 of FIG. 2 operates to increase or decrease the
loading on T3A to reduce or raise the LED current, respectively.
The controller 220 includes an operational amplifier (OP-AMP) U1
with a non-inverting input coupled to the output ground GND2 and an
inverting input receiving the LED current sensing signal from the
output rectifier 230 via a resistor R10. Resistors R9 and R10
provide a reference to the non-inverting input based on the level
set by a stabilized shunt regulator formed using zener Z8 and
capacitance C6, and the stabilized reference node at R9 is
connected via a resistor R8 to an upper terminal of a full bridge
rectifier D3-D6 coupled to the tertiary winding T3A. A zener Z6 is
coupled across the bridge D3-D6, and the loading of the bridge is
controlled by a MOSFET Q3 series connected with a zener Z7 across
the bridge output.
The amplifier U1 drives the gate of Q3 to perform proportional
integral (PI) control via integrator feedback capacitance C7 and
resistor R7 connected between the amplifier output and inverting
input to reduce any difference between the established reference
level and the LED current sense signal from the output rectifier
230. In general, the closed loop controller 220 thus increases the
loading (increasing the gate signal to Q3) to decrease the
inductance of the transformer windings T3B and T3C to thereby
increase the inverter frequency and thus decrease the LED output
current when the sensed LED current level is above the reference
level, and vice versa when the sensed LED current level is below
the reference level. The illustrated embodiment shows one exemplary
controller 220, but other embodiments are possible in which the
loading of a tertiary winding T3A is modified to control the output
of the rectifier 230. The DC-DC converter 200 thus provides
electrically isolated inverter input and rectifier output circuits
210 and 230 using the transformer T1 for isolated power transfer to
the output load, together with isolated control via the transformer
windings of T3, and self-oscillating operation using the resonant
circuit 213 and the gate drive circuit coupling via T2.
In one embodiment, the inverter 210 is provided with an input DC
bus level of about 450 volts at the terminals 122 and drives an LED
array at a nominal voltage of around 63 volts with the inverter
frequency being controlled in a range of around 120-160 kHz. In
this example, the following exemplary components and values may be
used: R1 and R2 10.OMEGA.; R3 and R4 2M.OMEGA.; R5 100 k.OMEGA.; R6
1 M.OMEGA.; R7 100.OMEGA.; R8 10 k.OMEGA.; R9 143 k.OMEGA.; R10 20
k.OMEGA.; R1 0.1.OMEGA.; C1 and C2 6.8 nF; C3 and C4 3.3 nF; C5 1
nF; C6 and C7 1 .mu.F; C8 2.times.47 .mu.F; T1A 2 mH; T1B 320
.mu.H; T2A 150 .mu.H; T2B and T2C 1.0 .mu.H; T3A 1 mH; T3B and T3C
500 .mu.H; D1 and D2 ED1F; D3-D6 1N4148; Z1 and Z3 TZM5239 9.1V; Z2
and Z4 TZM5250 20V; Z6 TZM5262 51V; Z7 TZM5231 5.1V; Z8 2.5V; Q1
and Q2 BSS165; Q3 BSS138; and U1 LMV931. By using the isolation
transformers T1 and T3, the installed LED array can have its
negative terminal connected to earth ground if desired, and this is
fully isolated from the supply main and the circuit ground of the
inverter 210 without requiring a separate power supply for
isolation. Moreover, the self-oscillating features of the inverter
210 allows for robust startup and steady-state operation without
requiring pulse width modulation (PWM) or frequency controller.
Referring also to FIG. 3, the DC-DC converter may further include
integral thermal protection incorporated into the isolated control
configuration as described above. In certain embodiments, the
transformer T3 has a ferromagnetic core 300 with a Curie
temperature Tc set at a temperature threshold level corresponding
to a desired thermal shutdown safety level for the DC-DC converter
200. For example, the threshold level may be set to 100 degrees C.
or 125 degrees C. or other level corresponding to a limit above
which one or more components of the converter 200 may be subject to
thermal failure or degradation. In this embodiment, once the
temperature of the core of T3 exceeds the threshold Tc, the core
material becomes paramagnetic and the inductance of the inverter
gate control circuit windings T3B and T3C decreases due to the
Curie effect, thereby increasing the inverter frequency. This
lowers the output power level, in turn reducing the temperature of
T3, and the closed-loop control will stabilize at a lowered power
operating point sufficient to prevent thermal degradation of the
DC-DC converter 200. Thus, even in high ambient temperature
operating conditions, the DC-DC converter 200 in this embodiment
provides thermal self-protection.
The above examples are merely illustrative of several possible
embodiments of various aspects of the present disclosure, wherein
equivalent alterations and/or modifications will occur to others
skilled in the art upon reading and understanding this
specification and the annexed drawings. In particular regard to the
various functions performed by the above described components
(assemblies, devices, systems, circuits, and the like), the terms
(including a reference to a "means") used to describe such
components are intended to correspond, unless otherwise indicated,
to any component, such as hardware, software, or combinations
thereof, which performs the specified function of the described
component (i.e., that is functionally equivalent), even though not
structurally equivalent to the disclosed structure which performs
the function in the illustrated implementations of the disclosure.
In addition, although a particular feature of the disclosure may
have been illustrated and/or described with respect to only one of
several implementations, such feature may be combined with one or
more other features of the other implementations as may be desired
and advantageous for any given or particular application.
Furthermore, references to singular components or items are
intended, unless otherwise specified, to encompass two or more such
components or items. Also, to the extent that the terms
"including", "includes", "having", "has", "with", or variants
thereof are used in the detailed description and/or in the claims,
such terms are intended to be inclusive in a manner similar to the
term "comprising". The invention has been described with reference
to the preferred embodiments. Obviously, modifications and
alterations will occur to others upon reading and understanding the
preceding detailed description. It is intended that the invention
be construed as including all such modifications and
alterations.
* * * * *